1. Field of the Invention
The present invention relates to a radio-controlled timepiece.
2. Description of the Related Art
Conventionally, there has been an electronic timepiece (radio-controlled timepiece) which includes functions to receive a radio wave (standard radio wave) including time information represented by a predetermined format (time code), to obtain time data, and to calibrate the time of an internal clock.
Among such radio-controlled timepieces, electromagnetic noise is generated by change of electric current or voltage in a driving signal of a liquid crystal in digital radio-controlled timepieces with a display using a digital display screen such as a liquid crystal display (LCD). When electromagnetic noise is mixed in receiving radio waves when the standard radio wave is received, the quality of the demodulated time code is damaged.
In view of the above, conventionally, techniques are developed in digital radio-controlled timepieces, such as a technique to pause driving of the liquid crystal while the standard radio wave is being received, or a technique to control operation so that the timing of discretely sampling the time code signal is different from the timing of driving the liquid crystal (for example, Japanese Patent Application Laid-Open Publication No. 2008-215929).
Moreover, since the electromagnetic noise generated by driving the digital display screen appears in higher order harmonic frequencies of the driving frequency, there is also a technique where the driving frequency is set so that the driving frequency of the digital display screen and its higher order harmonic frequencies do not overlap with the transmitted frequency of the standard radio wave (for example, Japanese Patent Application Laid-Open Publication No. 2012-242194 (corresponding to US 2012/0294126 A1).
Various differences occur in the oscillating frequency of the clock signal regarding driving the digital display screen due to characteristics of the oscillating circuit. In electronic timepieces, the time is kept by counting a clock signal or a predetermined frequency signal based on the clock signal. Therefore, if the oscillating frequency is different from the desired frequency, in other words, if there is an error, the accuracy of the time reduces according to the amount of deviation. Conventionally, in order to remove influence of such deviation, there are electronic timepieces which perform a logical slowing/quickening operation to thin input of signals to be counted in a percentage according to the deviation. The data which determines the percentage of thinning in the logical slowing/quickening operation is obtained in advance by testing before the electronic timepiece is shipped from the manufacturer, and the data is stored in the ROM.
However, usually, strict frequency accuracy is not required in the clock signal used for driving the digital display screen, and conventionally, signals including the deviation are used as they are. Therefore, even if the oscillating frequency is set based on the conventional techniques so that the oscillating frequency and its higher order harmonic frequencies are different from the receiving frequency of the radio wave including the time information, one of the oscillating frequency and its higher order harmonic frequencies overlaps with the receiving frequency due to the deviation of the oscillating frequency. Therefore, the signal of the time information may be damaged and it may not be possible to reliably receive the signal.